Form-associated custom elements: Refactor 'disabled' state handling code

Move 'disabled' state handling code from HTMLFormControlElement to
ListedElement. This is a preparation to support 'disabledStateChangedCallback'
of form-associated custom elmeents.

This CL has no behavior changes.

Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/187
Change-Id: I34703dd7b167be13e4b19324b5e931bda224a866
Reviewed-on: https://chromium-review.googlesource.com/c/1353039
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612032}
4 files changed